Currently, owing to the increasing worldwide energy demand, it is increasingly required to generate energy efficiently and cleanly in various systems, and this also applies to conventional systems for generating energy which are operated by combustion of hydrocarbons or hydrocarbon mixtures.
Gas-turbine operated generators in this case are a clean and efficient possibility for generating electrical energy, not only for public energy supply but also for industrial use. Thanks to the use of high-grade ceramic protective layers and sophisticated cooling concepts, gas turbines can now be operated with mean turbine intake temperatures of sometimes above 1500° C., and thus achieve net efficiencies of 40% without heat recovery and exergetic efficiencies of above 60% with heat recovery via coupling to a steam turbine. By using optimized burners, during operation with natural gas, mean nitrogen oxide emissions (NOx=total nitrogen oxide, i.e. NO and NO2 taken together) of less than 25 ppm (by volume) and carbon monoxide emissions (CO) of below 10 ppm can be guaranteed, wherein a defined test cycle is used as a basis for determination of the emissions. The national limiting values for power plant emissions may thereby be met in many industrial nations such as, e.g. Germany, still without any further exhaust gas aftertreatment.
Gas turbines are distinguished not only by low emissions and high efficiency, but also by capabilities such as operation under very differing loads (load flexibility from 100% down to values of below 30%) and rapid load changes up to rapid start (achieving the base load in less than 30 minutes). Because of the increasing fraction of energy from renewable sources (wind, solar, etc.) which are characterized by high fluctuations over time of the power fed into the electric supply grid, increasingly use is being made of the load flexibility of gas turbines. However, as a result, pollutant emissions occur which differ substantially from the average values determined in the test cycle: in the case of low burner outputs, the gas turbine burners are to be operated under equivalent conditions (i.e. fuel/air ratios based on the values required for stoichiometric combustion), which are far below the values required for the base load of the turbine.
Since these equivalent conditions are also far below the lean-burn limit for completely premixed combustion, increased emissions of uncombusted hydrocarbons and/or hydrocarbon mixtures (UHCs), of carbon monoxide (CO), and frequently also of nitrogen oxides, occur, and here there is a characteristic feature that the nitrogen oxides are emitted to an overwhelming proportion as nitrogen dioxide (NO2). The latter, from the viewpoint of discharge into the lower atmosphere, is actually not a problem, because nitrogen monoxide is also oxidized in the course of just a few minutes in the atmosphere to form nitrogen dioxide. However, NO2 has the unpleasant property of absorbing light in the blue and near-ultraviolet spectral range, in such a manner that even the operation of gas turbines, the pollutant emissions of which meet all legal provisions, owing to a yellow discoloration of the exhaust gas plume existing from the stack of the power plant in daylight, termed “yellow plume”), can lead to problems of acceptance.
In addition, in the case of gas turbines, there is frequently the possibility of using oil as an alternative fuel. In this case, compared with operation with natural gas, elevated nitrogen oxide emissions frequently occur, and the formation of NO2 in noticeable concentrations is observed over a broad range of loads.
Gas turbine power plants are long-term investments which to date—just as with coal power plants—have usually been operated at base load, in order to achieve the highest possible yields from the generation of electrical energy. The nitrogen oxide emissions in this case are composed of virtually 100% nitrogen monoxide (NO). Yellowish exhaust gas plumes have therefore not been observed except in the case of sporadically occurring start-ups of the gas turbine. Therefore, to date there have also been no problems with visible emissions which have only been initiated by the rapidly growing fraction of renewable energies in the energy market.
Likewise, nitrogen dioxide emissions and thereby yellowish discolorations can be formed in exhaust gases of further systems which, under certain operating conditions, burn hydrocarbons or hydrocarbon mixtures incompletely, such as, for example, gas-operated compressors, gas- or oil-operated boilers, gas engines or ships operated with diesel or heavy oil. The occurrence of these NO2 emissions in general and the dependence of this phenomenon on the fuel is due to the chemical gas-phase reactions between nitrogen monoxide (NO) and UHCs or CO in certain temperatures ranges which follow an incomplete combustion, as presented in MORIO HORI, NAOKI MATSUNAGA, NICK MARINOV, WILLIAM PITZ and CHARLES WESTBROOK: AN EXPERIMENTAL AND KINETIC CALCULATION OF THE PROMOTION EFFECT OF HYDROCARBONS ON THE NO—NO2 CONVERSION IN A FLOW REACTOR; Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute; 1998; pages 389-396. Not only during the start-up phase of, for example, gas turbines, but also during the operation in low partial load, however, the incomplete combustion of the fuel used is accepted, in order to be able to ensure stable operating behavior of the gas turbine.
However, the formation of NO2 can also have other causes which, in some circumstances, require different measures for reduction of NO2 emissions: thus, in the case of gas turbines having a waste-heat steam generator and integrated multistage exhaust gas aftertreatment consisting of upstream CO oxidation catalyst followed by ammonia (NH3) injection and downstream SCR catalyst (SCR, “selective catalytic reduction”), the formation of noticeable NO2 emissions was observed. To solve this problem, in EP 2 223 733 A1, the placing of the CO oxidation catalyst in a suitable temperature zone was proposed. Such an exhaust gas purification, however, is complex and costly in method terms, such that it is also not retrofittable at will, or is only retrofittable with considerable expense, in existing systems.